Revert of Disabling NOTREACHED which we're hitting flakily in browser tests.

webrtc/modules/video_coding/codecs/vp9/vp9_frame_buffer_pool.cc

Index: webrtc/modules/video_coding/codecs/vp9/vp9_frame_buffer_pool.cc
diff --git a/webrtc/modules/video_coding/codecs/vp9/vp9_frame_buffer_pool.cc b/webrtc/modules/video_coding/codecs/vp9/vp9_frame_buffer_pool.cc
index b8a2dcd800702a57c048b0446e0e1503e46ee2b2..62c05d34fa765bd4bc38e53b2a203c8c8070c87d 100644
--- a/webrtc/modules/video_coding/codecs/vp9/vp9_frame_buffer_pool.cc
+++ b/webrtc/modules/video_coding/codecs/vp9/vp9_frame_buffer_pool.cc
@@ -73,10 +73,7 @@
             << allocated_buffers_.size() << " Vp9FrameBuffers have been "
             << "allocated by a Vp9FrameBufferPool (exceeding what is "
             << "considered reasonable, " << max_num_buffers_ << ").";
-
-        // TODO(phoglund): this limit is being hit in tests since Oct 5 2016.
-        // See https://bugs.chromium.org/p/webrtc/issues/detail?id=6484.
-        // RTC_NOTREACHED();
+        RTC_NOTREACHED();
       }
     }
   }